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Predictability of El Niño-Southern Oscillation

Principal Investigator: Chris Landsea

Collaborating Scientist:

John Knaff (CIRA/Colorado State University)

Objective: To provide a baseline method for prediction of the El Niño-Southern Oscillation phenomena and to assess the skill of real-time predictions

Method: A statistical prediction method is developed for the El Niño -Southern Oscillation (ENSO) phenomena which is based entirely on the optimal combination of persistence, month-to-month trend of initial conditions and climatology. The selection of predictors is by design intended to avoid any pretense of predictive ability based on "model physics" and the like, but rather is to specify the optimal "no-skill" forecast as a baseline comparison for more sophisticated forecast methods. Multiple least squares regression using the method of leaps and bounds is employed to test a total of fourteen possible predictors for the selection of the best predictors, based upon 1950-1994 developmental data. A range of zero to four predictors were chosen in developing twelve separate regression models, developed separately for each initial calendar month. The predictands to be forecast include the Southern Oscillation (pressure) Index (SOI) and the Niño 1+2, Niño 3, Niño 4 and Niño 3.4 SST indices for the equatorial eastern and central Pacific at lead times ranging from zero seasons (0 - 2 months) through seven seasons (18 - 20 months). Though hindcast ability is strongly seasonally dependent, substantial improvement is achieved over simple persistence wherein largest gains occur for two to seven season (6 to 21 months) lead times. For example, expected maximum forecast ability for the Niño 3.4? SST region, depending on the initial date, reaches 92, 85, 64, 41, 36, 24, 24 and 28 percent of variance for leads of zero to seven seasons. Comparable maxima of persistence only forecasts explain 92, 77, 50, 17, 6, 14, 21 and 17 percent, respectively. Details on the paper can be found in Knaff and Landsea (1997).

The very strong 1997-98 El Niño was the first major event in which numerous forecasting groups participated in its real-time prediction. A previously developed simple statistical tool - the El Niño-Southern Oscillation CLImatology and PERsistence (ENSO-CLIPER) model - is utilized as a baseline for determination of skill in forecasting this event. Twelve statistical and dynamical models were available in real- time for evaluation. Some of the models were able to outperform ENSO- CLIPER in predicting either the onset or the decay of the 1997-98 El Niño, but none were successful at both for a medium-range two season (6-8 months) lead time. There were no models, including ENSO-CLIPER, able to anticipate even one-half of the actual amplitude of the El Niño's peak at medium-range (6-11 months) lead. In addition, none of the models showed skill (i.e. lower root mean square error than ENSO- CLIPER) at the zero season (0-2 months) through the two season (6-8 months) lead times. No dynamical model and only two of the statistical models (the canonical correlation analysis [CCA] and the constructed analog [ANALOG]) outperformed ENSO-CLIPER by more than 5% of the root mean square error at the three season (9-11 months) and four season (12-14 months) lead time. El Niño impacts were correctly anticipated by national meteorological centers one half year in advance, because of the tendency for El Niño events to persist into and peak during the Boreal winter. Despite this, the zero to two season (0-8 month) forecasts of the El Niño event itself were no better ENSO-CLIPER and were, in that sense, not skillful - a conclusion that remains unclear to the general meteorological and oceanographic communities. Details on this analysis are found in Landsea and Knaff (2000).

A continuing effort is being made to periodically revise the assessment of skill in real-time predictions of El Niño-Southern Oscillation by groups from around the world. An evaluation of the 1998-2001 La Nina event and the onset of the 2002-03 El Niño event is forthcoming.

Accomplishments:

  • ENSO-CLIPER program, documentation and archive of forecasts are available (as of 2000) on the web.
  • Real-time forecasts from ENSO-CLIPER were published in the Experimental Long-Lead Forecast Bulletin quarterly (1997-2001).
  • Real-time forecasts from ENSO-CLIPER are being published quarterly in the Climate Diagnostics Bulletin (2002-03).

References:

Knaff, J. A. and C. W. Landsea, 1997: An El Niño-Southern Oscillation Climatology and Persistence (CLIPER) forecasting system. Wea. Forecasting, 12, 633-652.

Landsea, C. W. and J. A. Knaff, 2000: How much skill was there in forecasting the very strong 1997-98 El Niño? Bull. Amer. Meteor. Soc., 81, 2107-2119.


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Last modified: 02/21/2003

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